For theatrical lighting, and for various applications of specialized commercial lighting, it is a common practice to utilize special lighting fixtures occasionally track mounted but more frequently mounted in other ways, which can be aimed and focused on a particular area. Such lighting fixtures typically incorporate a relatively high intensity light source, one or more focusing lenses, and provisions to accommodate accessory cartridges with filters, etc. for achieving a variety of specific lighting effects. Many such lighting fixtures also incorporate adjustable framing shutters that are movable to positions partially blocking and shaping the light source. Typically, there are a plurality of such shutters, for example four, spaced around the circumference around the lighting fixture. These are adapted to be manually adjusted inward and outward with respect to the center line of the lighting fixture, and also to be tilted, all for the purpose of enabling the light discharged from the fixture to be rather precisely shaped and confined, such that it may be directed with a considerable degree of precision at a particular object area to be illuminated.
Framing shutters for lighting fixtures of the type described above advantageously can include a shutter panel, which projects more or less radially into the interior of the lamp housing. The shutter is formed of relatively thin sheet metal and typically has an integral extension arm projecting generally radially outward of the lamp housing and mounting a handle at its outer end. Friction elements act on the shutter plates, tending to hold them in any position in which they are set.
In a typical setup operation, a lighting technician will aim and focus the lighting fixture and, in conjunction therewith, will shift the framing shutters radially inward or outward, and perhaps tilt one or more of them in their planes, with the end result that the focused beam is both confined and shaped in a manner calculated to achieve a desired effect.
In many, if not most, situations, once a lighting fixture has been aimed, focused and the light beam suitably shaped and confined with the framing shutters, it may be expected to remain in that setting for an extended period of time (e.g., weeks or months). In many cases, however, the lighting fixture may be subject to bumping, jarring, vibration, etc., which can have the effect of shifting one or more of the framing shutters from its originally adjusted position. This can occur over a period of time, as where the fixture may be subject to a vibrational environment, or it may happen more abruptly if a handle of a framing shutter is accidentally bumped.